1. Field of the Invention
This invention relates to an exhaust gas recirculation (EGR) system for an internal combustion engine, and more particularly to an improved EGR system in which the amount of the recirculated exhaust gas is kept at an optimal value even when the exhaust gas recirculating passage clogs.
2. Description of the Prior Art
There has been known an EGR system in which a part of the exhaust gas is mixed with the air-fuel mixture by way of an exhaust gas recirculating passage communicating the exhaust passage with the intake passage to reduce the amount of NOx formed in the combustion process of the engine.
When the exhaust gas is mixed with the air-fuel mixture, the amount of the inert component in the air-fuel mixture introduced into the engine is increased and the combustion temperature is lowered, whereby the amount of NOx is reduced. However, if an excessive amount of exhaust gas is mixed with the air-fuel mixture, there arise problems that the engine power is lowered, the amount of unburned gasoline (HC) in the exhaust is increased and the combustion process becomes unstable. Therefore, the amount of the recirculated exhaust gas must be set at an optimal value taking into account these conditions. Thus, the exhaust gas recirculating passage is generally provided with a flow control valve for controlling the amount of the recirculated exhaust gas according to the operating condition of the engine.
However, after long operation of the engine, carbon contained in the recirculated exhaust gas deposits in the exhaust gas recirculating passage to clog it and reduce the effective cross-sectional area of the passage. This reduces the flow rate of the recirculated exhaust gas for a given opening degree of the flow control valve in the exhaust gas recirculating passage so that the desired NOx-reducing effect cannot be obtained
In Japanese unexamined patent publication No. 57(1982)-41454 is disclosed an EGR system in which a pressure sensor is provided in the exhaust gas recirculating passage downstream of the flow control valve therein to detect the degree of clogging of the passage by way of the pressure detected by the pressure sensor, and the opening area of the flow control valve for a given operating condition is increased according to the degree of clogging. However this EGR system is disadvantageous in that the structure associated with the exhaust gas recirculating passage is inherently complicated.